In a conventional non-contact power transmission device which includes a primary side unit and a secondary side unit of a coupling transformer which house a primary side and a secondary side individually and can be separated from each other, it is required that power transmission efficiency is improved without upsizing the device.
In order to achieve the above object, conventionally a coupling coefficient of a non-contact transformer is solely increased (for example, see Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-199598
Patent Document 2: Japanese Patent Application Laid-Open No. 2000-269058
However, when a gap between a primary core and a secondary core of the coupling transformer is close to 10 mm, it is not easy to increase the coupling coefficient. This is the first disincentive at the time of realizing high-output power non-contact power transmission devices.
Further, when a parallel resonance circuit which is conventionally used as a secondary side resonance circuit of the coupling transformer in the non-contact power transmission device is used, as load impedance is lower, the power which can be supplied to the load becomes less. This is the second disincentive to realizing high-output power of the non-contact power transmission devices in the lower impedance load.
This will be explained with reference to FIG. 6.
In an equivalent circuit where a secondary side parallel resonance circuit and a load are connected as shown in FIG. 6, an electric current I2 flowing in a load RL connected to the resonance circuit in parallel is expressed by the following formula when an angular frequency is designated by ω, a load of the secondary side circuit is designated by QL, a coil current of the primary side circuit is designated by I1, a mutual inductance of the coupling transformer is designated by M and an inductance of the secondary side coil is designated by L2.
[Mathematical Formula 1]I2=(ω×M×I1×QL)/RL  (Formula 1)Here,
[Mathematical Formula 2]QL=RL/(ω×L2)  (Formula 2)
For this reason, QL of the mathematical formula 2 is assigned to the mathematical formula (1), and the following formula is obtained:
[Mathematical Formula 3]I2=(M×I2)/L2  (Formula 3)
According to the (formula 3), when the primary current I1 is constant, the load current I2 becomes constant regardless of the load RL. The secondary side is expressed equivalently by a constant-current power supply. For this reason, even if the load is reduced, the electric power which can be supplied to the load reduces because the load current is constant.
Further, when the primary side of the coupling transformer is driven by a rectangular wave, a harmonic component is unnecessarily emitted from the primary side coil L1 of the coupling transformer to a space, thereby possibly interfering peripheral electronic devices.
In the non-contact power feeding device, therefore, a circuit in which a low-pass filter is connected to a front side of the primary side feeder cable is proposed in order to reduce a harmonic current of a primary side feeder cable (primary side coil) (for example, see Patent Document 3).
Patent Document 3: Japanese Patent Application Laid-Open No. 11-224822